The invention pertains to a crystal holding device for use during the process of growing crystals by the Czochralski drawing method.
Apparatuses for growing single crystals by means of the Czochralski method are known. In apparatuses of this type, the crystal to be grown starts out as a seed crystal immersed in a melt and is grown either by lowering the melt bath or by raising the seed crystal up out of the melt. The single crystals drawn in this way out of a melt of silicon, for example, form the starting material for the production of so-called "wafers".
By means of the known apparatuses, it is has been possible over the course of recent years, through continuous improvements to the growing process and to the apparatuses used for this purpose, to increase the weights of the crystals to as much as 300 kg. It has turned out to be extremely difficult to increase the diameter of the single crystals produced by the Czochralski method. A basic reason for this is that, upon conclusion of the drawing process, the entire weight of the crystal is held by the seed crystal, which, because of the nature of the process, has a diameter of only 4-5 mm. It is necessary, however, to prevent the crystal from breaking off from the seed crystal and from thus destroying the crystal drawing apparatus. But because the tensile stress to which a seed crystal can be subjected cannot exceed a certain maximum value, only crystals of limited dimensions with a limited maximum weight can be produced in the known crystal drawing apparatuses.
Another problem which occurs during the drawing process is that, because the crystal is rotated as it grows, notch stress builds up in the seed crystal, which further reduces the maximum amount of load which the single crystal can be allowed to exert on the seed crystal. To guarantee a reliable growth process, in which the crystal is prevented from breaking off from the seed crystal, the process parameters during the drawing process are necessarily chosen in such a way that the tensile stress acting on the seed crystal remains significantly below the maximum allowable tensile stress.
The known crystal growing apparatuses thus suffer from the significant disadvantage that total weight and thus the crystal volume of the crystals which can be produced are limited. This disadvantage limits the economy of production by means of the known crystal drawing apparatuses.
The task of the present invention consists in making available an apparatus by means of which crystals can be produced by the Czochralski method in such a way that the crystal volume or crystal weight can be greater than the maximum weight determined by the amount of the tensile stress which can be exerted on the seed crystal.